How do scientists study abrupt climate changes of the past?

In the 19th century, humans began to use modern instruments such as thermometers and rain gauges to record weather and climate. To reconstruct variations in the climate system earlier in time, it is necessary to use natural climate recorders, such as ice cores, tree rings, ocean and lake sediments, and corals. Measurements collected from these natural climate archives are called "proxies" because they do not provide a direct measurement of climate, as a thermometer or rain gauge does. Rather, scientists use them to make inferences about past climate changes.

The ring width of a tree is an example of a proxy for temperature, or in some cases rainfall, because the thickness of the annual ring is sensitive to the temperature and/or rainfall of that year. The most valuable proxies are those that can provide numerical values of temperature or rainfall amount, along with estimates of their uncertainty. Proxies that cannot be quantified in terms of climate variables are less precise because the magnitude of climate change cannot be determined.

A key step in the study of abrupt climate change is to date the samples. For natural archives with clear annual layers, such as trees and some ice and sediment cores, it is possible to count the layers back in time. Another useful method for many natural climate recorders is to measure radioactive isotopes such as radiocarbon (14C) whose decay forms a precise clock marking long intervals of time.

If we know the time between different samples and the magnitude of change observed in the proxy, we can calculate the rate of change. Uncertainties in the climate reconstruction from proxies and uncertainties in proxy dating are the main reasons that abrupt climate change is one of the more difficult topics to study in the field of paleoclimatology.

For more on how different types of proxies can provide insights into past climatic and environmental conditions, see the summary of Paleo Proxy Data.